1. Field of the Invention
The present invention relates to a semiconductor liquid phase epitaxial growth method and apparatus for forming a semiconductor layer on a semiconductor wafer on the basis of epitaxial growth by use of an epitaxial layer growing liquid source, and a wafer holder for accommodating and holding wafers for the epitaxial growth.
2. Description of the Prior Art
The liquid phase epitaxial growth method is such a technique that a source is formed by dissolving a semiconductor material for epitaxial growth in a liquefied low-melting point metal used as a solvent; the formed source is brought into contact with a semiconductor wafer; and then the temperature of the source is lowered to deposit the dissolved semiconductor on a wafer as an epitaxial growth layer. This technique is adopted to obtain an epitaxial growth layer of a compound semiconductor such as GaP (gallium phosphide), GaAs (gallium arsenide), AlGaAs aluminum gallium arsenide), etc. by use of Ga (gallium) as a solvent or to obtain an epitaxial growth layer of a Si (silicon) epitaxial growth layer by use of Ga (gallium) or Sn (tin) as a solvent.
In principle, the liquid phase epitaxial growth method comprises a step of bringing the source into contact with a wafer; a step of lowering the temperature of the source; and a step of separating the source from the wafer where necessary. According to the method of bringing or separating the source into contact with or from the wafer, various liquid phase epitaxial growth methods and apparatus have been so far known, for instance such as source injection method, dipping method, slide boat method, etc.
In the case of the source injection method, wafers are accommodated in a wafer holder and then disposed in an epitaxial growth chamber, and then the source is injected into the epitaxial growth chamber for epitaxial growth.
An example of the prior art source injection method and apparatus will be explained hereinbelow with reference to FIGS. 14(a), 14(b) and 14(c).
The epitaxial growth apparatus as shown in the drawings is generally referred to as a boat, and used in a horizontal or vertical furnace. FIGS. 14(a) to 14(c) show the case where the boat is used in a horizontal furnace. FIG. 14(a) shows the state where the boat is placed in the furnace before epitaxial growth, in which, however, the furnace and quartz tube are all not shown. In FIG. 14(a), the boat is mainly composed of a body 5501, a source sump 5502, and an exhaust source sump 5503. Further, a partition plate 5504 is disposed between the body 5501 and the exhaust source sump 5503. Further, an epitaxial growth chamber 5506 for accommodating wafers 5505 for epitaxial growth is formed in the body 5501. The source sump 5502 and the partition plate 5504 are both slidable in the horizontal direction, so that these elements can be moved right and left during the epitaxial growth process by use of a quartz rod inserted into the quartz tube from the outside.
For the epitaxial growth, as shown in FIG. 14(a), wafers 5505 are mounted in the epitaxial growth chamber 5506. As shown in FIG. 14(b), the wafers 5505 are held vertically, or obliquely with respect to the horizontal direction. Further, in FIGS. 14(a) to 14(c), a mechanism (a wafer holder) for holding the wafers is not shown. An epitaxial source raw material is put in the source sump 5502. In the case of the GaAs epitaxial growth, for instance, the epitaxial source raw material is metal Ga (as the solvent), poly GaAs (as the epitaxial growth material), and Si (as dopant), which are all are added into the source sump.
Under these conditions, when the boat is introduced into the furnace and then the furnace temperature is raised to such a high temperature that the source raw material metal Ga can be melted and thereby a source 5507 saturated with the dissolved epitaxial growth raw material can be obtained. To obtain the source 5507, a relatively long time (longer than one hour, in general) is necessary. When the source 5507 has been obtained, as shown in FIG. 14(b), the source sump 5502 is slid to the right or left side in the horizontal direction to fit a hole 5081 formed in the source sump 5502 to a source passage 5509. By doing this, the source 5507 can flow into the epitaxial chamber 5506 through the source passage 5509, so that the source is brought into contact with the wafers 5505. In the state where the source 5507 is in contact with the wafers 5505, when the temperature is lowered, the epitaxial growth raw material over-saturated in the source 5507 is deposited on the wafers as an epitaxial growth layer, respectively.
At a time when an epitaxial growth layer of any desired amount has been obtained, as shown in FIG. 14(c), the partition plate 5504 is slid to the right or left side in the horizontal direction to fit a hole 5085 formed in the partition plate 5505 to an exhaust hole 5083 formed in the epitaxial growth chamber 5506. By doing this, the used epitaxial source 5507 can drop into the exhaust source sump 5503, so that the epitaxial growth process ends.
In FIGS. 14(a) to 14(C), the epitaxial growth method and apparatus for forming only a single epitaxial layer have been explained. In the source injection method, generally, after the source used for the epitaxial growth has been exhausted, a new source is injected again for an additional epitaxial layer, to obtain a multi-layer epitaxial growth layer.
FIG. 15 shows an example of the apparatus used for the two-layer epitaxial growth. A source sump 6602 is partitioned right and left, as a first source 6071 and a second source 6072. Each of the separated source sumps is formed with each inlet port 6081 and 6082. Therefore, when the source sump 6602 is moved to the right side and the left side, it is possible to obtain two epitaxial growth layers continuously, by first injecting the raw material of the first source 6071 disposed on the left side, by exhausting the first source, and by injecting the raw material of the second source 6072 disposed on the right side.
In the case of the dipping method, a source sump in which a source raw material is put is placed in a heating furnace; the source raw material is dissolved by heat as an epitaxial growth source; and then wafers are dipped into the source. After that, the temperature of the heating furnace is lowered for epitaxial growth. After the epitaxial growth of any desired film thickness has been obtained, the wafers are pulled upward from the source to complete the epitaxial growth process.
In the case of the slide boat method, wafers are held on a wafer holder; a slider having a source is placed on the wafers and then heated. After the source has been heated up to a predetermined temperature, the slider is moved to bring the source into contact with the wafers for epitaxial growth.
In the above-mentioned prior art liquid phase epitaxial growth methods, however, there exist some problems related to the principle. These problems will be described in further detail hereinbelow in association with the epitaxial growth methods and the wafer holder for holding the wafers.
(1) Polycrystal deposition and film thickness non-uniformity
In the liquid phase epitaxial growth method, an over-saturated solute is deposited and grown on a wafer. Here, the solute can reach the wafer being moved in the source on the basis of diffusion caused by a concentration gradient generated by deposition. Therefore, the solute of the source located far away from the wafer cannot reach the wafer, with the result that the solute is deposited as polycrystal in the source or on the wall surface of the boat.
Here, since the polycrystal is grown non-uniformly, when the solute is deposited in the source near the epitaxial growth surface of the wafer or the opposite surface or the peripheral surface of the wafer, the epitaxial growth is prevented by the polycrystal deposition of the polycrystal, with the result that the film thickness of the epitaxial growth is not uniform.
In the injection method, although the source passage for introducing the source from the source sump to the epitaxial growth chamber is essential, since the polycrystal is easy to be deposited in the source passage, the deposited polycrystal prevents the source from flowing through the source passage. This phenomenon causes a serious problem when after the first semiconductor layer has been grown, the source is exchanged for another source for the second semiconductor layer.
In the dipping method, the polycrystal deposited on the outer edge or the peripheral portion of the wafer causes a problem. When the epitaxial layer is grown by simply dipping the wafer into the source, it is a matter of course that the solute is deposited and grown on the reverse surface of the wafer. In order to overcome this problem, Japanese Published Unexamined Patent Application No. 4-160092 discloses such a method as shown in FIGS. 16(a) and 16(b). In this prior art method, each of wafers 14 is placed in each of laboratory dishes 13; a plurality of the dishes are stacked upon each other and further fixed by use of a cassette 15; and then the cassette 15 is placed in the source 12 for epitaxial growth.
In this method, since the reverse surface of each wafer is brought into tight contact with the dish surface (without being brought into contact with the source), an epitaxial growth is not obtained on the reverse surface of the wafer. Further, when a plurality of the dishes are used, it is possible to obtain an epitaxial growth layer on a plurality of the wafers, respectively at a time.
On the other hand, since the epitaxial growth is performed by holding the wafers on the dishes horizontally, after the epitaxial growth has been completed, when the wafers and the wafer holders are pulled out of the source, since the source remains on the upper surfaces of the wafers, it is impossible to end the epitaxial growth quickly. As a result, it is impossible to control the film thickness of the semiconductor layer precisely. In addition, since the amount of the remaining source is not uniform, the thickness of the epitaxial growth layer disperses among the wafers and additionally on the surface of the same wafer. Further, the flatness of the surface of the epitaxial growth layer is degraded.
Further, since polycrystal is easily deposited on the bottom surface of the dish which is opposed to the upper surface of the wafer, there exists another problem in that the film thickness of the epitaxial growth layer is not uniform.
To overcome this problem, Japanese Patent Laid-Open No. 60-21897 discloses such a method as shown in FIG. 17. In this dipping method, each of the wafer reverse surfaces is brought into tight contact with a spacer.
In this dipping method, however, nothing is considered of the diffusion of the solute from the periphery of the wafer and the deposition of the solute onto the wafer edge portion. In more detail, FIG. 18 shows a state where wafers 1905 are dipped into a source 1907 for epitaxial growth, in which since the reverse surfaces of the wafers 1905 are brought into tight contact with two spacers 1911, respectively, the reverse surfaces of the wafers 1905 are free from epitaxial growth. In this case, however, as already explained, in the liquid phase epitaxial growth, since the solute in the source contacting with the surface of the wafer diffuses onto the wafer edge, epitaxial growth occurs. As shown in FIG. 18 simply, at the central portion of the wafer 1905, only the source existing between the two opposing wafers contributes to the epitaxial growth. In contrast with this, in the edge portion of the wafer 1905, the source existing at the periphery of the wafer also contributes to the epitaxial growth. As a result, since the thickness of the epitaxial growth becomes larger at the edge portion than at the central portion thereof, there exists a problem in that the thickness of the epitaxial growth layer is not uniform on the same surface of the wafer, so that the uniformity of the intra-surface film thickness deteriorates.
Further, in general, since the edge portion of the wafer is chamfered, the crystal orientation is different between the central portion and the edge portion of the wafer surface. In addition, since the wafer surface is rough, there exists another problem in that abnormal epitaxial growth (e.g., polycrystal growth) easily occurs at the chamfered edge of the wafer.
Further, in the case of the dipping method, since the solute is deposited in the source within the source sump and further since the solute is more light than the solvent Ga, the deposited solute floats on the surface of the solvent. Therefore, the solute (e.g., polycrystal GaAs) deposited under the wafer holder during the epitaxial growth adheres onto the wafer or the wafer holder in the source. Further, the solute floating on the source surface during the epitaxial growth adheres onto the upper portion of the wafer, when the wafer is being pulled upward after the end of the epitaxial growth.
As described above, in the Japanese Patent Laid-Open No. 60-21897, nothing is considered of the abnormal growth at the peripheral portion of the wafer, of the non-uniform film thickness of the epitaxial growth, and of the solute deposited in the source.
(2) Thermal deterioration of wafer
Since the vapor pressure of As is high, when GaAs wafer is exposed to a high temperature, As is vaporized from the surface of the GaAs wafer, so that the surface thereof deteriorates. To prevent the vaporization of As, it is necessary to place the wafer in as airtight and narrow a space as possible, in order to prevent As from vaporization by increasing the As vapor pressure at the wafer periphery, without placing the wafer at high temperature for many hours. In the heat treatment of the GaAs wafer, for instance, a cap anneal method is adopted such that an oxide silicon film is formed on the surface of the wafer to prevent As form being vaporized.
In the case of the liquid phase epitaxial growth, however, since a long time and a high temperature (higher than that for epitaxial growth) are required to dissolve the source raw material, the wafer is exposed to a high temperature, even at the preparatory stage before the epitaxial growth begins. Further, since the epitaxial surface of the wafer must be brought into contact with the source, it is impossible to perfectly cover the wafer surface.
In the injection method, since the wafer is held within the boat located under the source and then inserted into the high temperature section of the furnace together with the source raw material, the wafer is exposed to the high temperature for a further long time. In addition, since the source injecting passage communicates with the epitaxial growth chamber in which the wafer is placed, it is impossible to air-tightly seal the wafer within a narrow space.
In the dipping method, it is possible to place the wafer at a low temperature section prepared inside or outside the heating furnace, until the preparatory process of the epitaxial growth has been completed. However, immediately before the dipping, since the wafer must be preheated up to near the same temperature as that of the source, it is impossible to prevent the thermal deterioration at this stage.
Japanese Patent Laid-Open No. 60-115271 discloses a method of placing wafers at a low temperature section with respect to the epitaxial growth of GaP. In this method, during preheating, the wafer is placed over the liquid surface of the source within a source sump, so that the wafer can be closed tightly by placing a lid onto the source sump.
In this disclosed method, however, the source sump is closed by use of a lid, only to prevent a dopant source (i.e., oxygen) from being vaporized from the epitaxial growth source. Therefore, since the space within the source sump is large, it is impossible to perfectly prevent the wafer from thermal deterioration.
Further, in the method of placing the wafer at a low temperature portion outside the heating furnace, the temperature gradient within a reaction tube in which a substrate is placed drops abruptly, beginning from the portion of the reaction tube extending from the furnace to the outside. Therefore, there exists a problem in that when the substrate is placed outside the furnace, a large temperature distribution occurs on the surface of the substrate, so that a thermal stress is generated. Further, when the wafers are moved downward to a high temperature section within the furnace, since a large temperature difference occurs, there exists a problem in that a thermal stress occurs. Once the thermal stress occurs, the substrate is warped, so that the crystal defect may occur.
(3) Non-uniform composition of epitaxial growth source
For instance, in the case of the liquid phase epitaxial growth of GaAs, the epitaxial growth source is obtained by putting the solvent metal Ga, the epitaxial growth material of poly GaAs, and the dopant of Si into a source sump and then by increasing the temperature of the source sump. In this process, it is difficult to obtain a uniform solvent on the basis of only the natural diffusion. This is because since the As is more light in weight than Ga, the concentration of As increases near the surface of the source, so that Ga tends to be non-saturated at the bottom portion of the source sump. Further, in the case of the epitaxial growth raw material containing a small amount of dopant, there exists a problem in that the source tends to be non-uniform. This is because the epitaxial growth raw material is supplied in the form of solid, it is impossible to place the raw material uniformly within the source sump.
To overcome this problem, it may be considered that the source is stirred within the source sump. However, in the aforementioned boat used in the horizontal furnace, since the stirring motion must be made in the horizontal direction, it has been difficult to arrange a stirring device or mechanism.
Further, in the dipping method using a vertical furnace, although it is relatively easy to arrange the stirring mechanism, there exists a problem with respect to the saturation degree of the source during the wafer dipping process. If the source is not saturated, there exists a problem in that immediately when the source is brought into contact with the wafer, the solute is dissolved. This problem occurs, irrespective of the epitaxial growth method. However, when the source is over-saturated, the deposited solute covers the wafer surface. Therefore, in the dipping method, in particular there exists such a restriction that the source must not be kept over-saturated.
(4) Residual and mixing of epitaxial growth raw material
When a multi-layer epitaxial growth (i.e., two or more layers) is made by the injection method, after the first source has been exhausted, the second source is injected. In this case, however, polycrystal of the first source is deposited and adhered onto the wafer. In the source injection method in particular, since there is a large space in the source passage, a source flows in larger quantities than those required for the epitaxial growth, the above-mentioned phenomenon becomes more conspicuous. The adhered substance cannot be exhausted even in the source exhaust process, and thereby remains within the boat body together with the source containing the adhered substance. Therefore, when the second source is injected, the second source is inevitably mixed with the first source.
For instance, when AlGaAs epitaxial growth is made after the GaAs epitaxial growth, since the AlGaAs source is mixed with GaAs source, there arises a problem in that the mixed crystal ratio of Al in the AlGaAs layer is reduced or becomes non-uniform or that the dopant concentration cannot be controlled.
In the case of the dipping method, the multi-layer epitaxial growth is made by preparing a plurality of sources and source sumps and by dipping the wafer into the sources in sequence. In the aforementioned Japanese Patent Laid-Open No. 4-160092, as shown in FIG. 16, since the wafer is accommodated in the dish-shaped wafer holder, it is difficult to exhaust the source. Therefore, the source remaining on the wafer is easily mixed with another source, so that the source composition is easily changed, thus causing a problem in that it is difficult to obtain an epitaxial growth layer of desired film thickness and composition.